Conventionally, refrigeration apparatuses such as air conditioners, in which chlorofluorocarbon or an alternative for chlorofluorocarbon is used as a refrigerant, are widely used. However, these refrigerants are responsible for the problems such as ozone depletion and global warming. In view of these, air conditioners have been proposed in which water is used as a refrigerant having a very low impact on the global environment. As an example of such an air conditioner, Patent Literature 1 discloses an air conditioner designed specifically for cooling a room.
When water is used as a refrigerant, a large amount of refrigerant vapor needs to be compressed at a high compression ratio. Accordingly, the air conditioner disclosed in Patent Literature 1 includes two compressors, i.e., a centrifugal compressor and a positive displacement compressor, and these compressors are arranged in series so that a refrigerant vapor compressed by the centrifugal compressor is further compressed by the positive displacement compressor.
In addition, when water is used as a refrigerant, the temperature of the refrigerant discharged from a compressor is high due to the physical properties of water. Therefore, the durability of members constituting a high-pressure part of an air conditioner decreases. In order to address this problem, it is effective to dispose a vapor cooler between the upstream-side compressor and the downstream-side compressor as in the air conditioner disclosed in Patent Literature 1, so as to temporarily lower the temperature of the refrigerant vapor in the course of the compression process.